1. Field of the Invention
The present invention relates to the separation of ferric iron and the recovery of aluminum fluoride trihydrate (AlF.sub.3.3H.sub.2 O) from aqueous aluminum fluoride solutions and particularly from such solutions which contain significant amounts of dissolved ferric iron.
2. Description of the Prior Art
Aluminum fluoride trihydrate (AlF.sub.3.3H.sub.2 O) is used in the production of aluminum fluoride (AlF.sub.3), usually by a heating operation that drives off the water of hydration. The aluminum fluoride, in turn, can be used, for example, in the manufacture of cryolite (Na.sub.3 AlF.sub.6), in the manufacture of aluminum (Al) by the cryolite process, and in the manufacture of alumina (Al.sub.2 O.sub.3) as described, for example, in our aforementioned U.S. Pat. No. 3,961,030, the disclosure of which is incorporated herein by reference. A common method of making AlF.sub.3.3H.sub.2 O is by the crystallization of aqueous AlF.sub.3 solutions. This crystallization is carried out in retention tanks where the aluminum fluoride solutions are agitated for 2-6 hours at temperatures anywhere between 140.degree. and 212.degree. F. under atmospheric pressure in a continuous operation. A batch operation may also be employed, and vacuum crystallization, instead of atmospheric, can be used. The crystallization results in the formation of AlF.sub.3.3H.sub.2 O crystals which are separated from the remaining solution, or "mother liquor", by filtration or any other suitable means.
When the aluminum fluoride solution from which AlF.sub.3.3H.sub.2 O is crystallized is obtained from a ferric iron-containing clay, or when for whatever reason the solution fed to the crystallizers is contaminated with ferric iron, a substantial portion of the ferric iron reports in the final product of the process. This is undesirable, of course, because commercial specifications are such that the AlF.sub.3.3H.sub.2 O stream, after filtration, may not contain more than about 0.1% by weight ferric iron, expressed as Fe.sub.2 O.sub.3, and quite frequently no more than 0.05% by weight ferric iron. Even though most of the ferric iron in the feed to the crystallizers tends to remain with the mother liquor, these commercial specifications are almost impossible to meet when the feed to the crystallizers contains more than about 1 gpl (grams per liter) Fe.sub.2 O.sub.3, and especially when it contains more than about 2 gpl Fe.sub.2 O.sub.3, since substantial amounts of the ferric iron are retained with the aluminum fluoride crystals and remain with them in subsequent steps of the process.
U.S. Pat. No. 3,320,032 (Feller) describes the extraction of iron from solutions containing aluminum sulfate, chloride or nitrate but avoids any mention or suggestion of aqueous aluminum fluoride solutions. Furthermore, the extractants used in the Feller patent exclude the mono(alkylphenyl)phosphoric acids or mixtures of same with di(alkylphenyl)phosphoric acids.
U.S. Pat. No. 3,835,214 (Hurst et al) extracts uranium from "wet-process" phosphoric acid solutions, resulting from the acidulation of phosphate rock and containing phosphoric acid and uranium, iron, calcium, sulfate and fluoride ions, using a mixture of mono- and di(octylphenyl)phosphoric acid (OPPA) as the extractant. The uranium transfers to the extractant but iron apparently does not, thus, providing another example of a prior art teaching which leads the skilled worker away from applicants' invention. We are not here concerned with the extraction of uranium but with the extraction of iron.
U.S. Pat. No. 3,211,521 (George et al) discloses the removal of iron from aqueous aluminum sulfate or nitrate, but makes no mention of the aluminum fluoride solutions. George et al, furthermore, does not disclose, teach or suggest the mixture of mono- and di(octylphenyl)phosphoric acids found to be so unique in the present invention in the extraction of ferric iron from aqueous aluminum fluoride solutions.
The Marcus et al text entitled "Ion Exchange And Solvent Extraction Of Metal Complexes", Wiley-Interscience, Div. of John Wiley And Sons Ltd., New York, 1969, pages 523-525, 534-536, 538, 546 and 550, provides a general discussion of the extraction of metals from certain types of aqueous solutions of aluminum chloride or aluminum sulfate but fails to mention aluminum fluoride aqueous solutions. The extractants disclosed in Marcus et al are alkyl phosphoric acid derivatives and a (2-ethylhexyl)phenylphosphoric acid but no mixture of a mono- and a di(alkylphenyl)phosphoric acid is disclosed. As a matter of fact, the reference (page 525, lines 9-28) teaches the desirability of purifying phosphoric acid mixtures to separate the phosphoric acids because impurities "affect greatly the extractive properties of the reactants". The reference goes on to state that "Much attention, has, thus, been given to the separation of the acids and their purification". The Marcus reference, therefore, tends to lead the skilled worker away from using mixtures and encourages the separation of the acids and purification thereof.
Peppard et al, Journal of Inorganic and Nuclear Chemistry, 1958, Vol. 7, pp. 269, 276-285, presents a general study of the metal extraction power of mixtures of certain esters of orthophosphoric acid, but fails to suggest or disclose the selective extraction of ferric iron from aqueous aluminum fluoride solutions containing it.
U.S. Pat. No. 3,966,909 (Grunig et al I) discloses the process for the recovery of aluminum from aqueous aluminum nitrate, sulfate or chloride solutions. There is no disclosure or suggestion of the extraction of iron from aqueous aluminum fluoride solutions or the use of a combination of mono- and di-(alkylphenyl)phosphoric acids. In addition, the process of Grunig et al I achieves an effect that is quite opposite to that achieved by the present invention, i.e., it removes aluminum in the extractant and leaves most of the ferric iron impurity in the aqueous solution and thus teaches away from the present invention.
U.S. Pat. No. 3,729,541 (Grunig et al II) teaches that commercial alkylphenyl phosphoric acids sometimes come in mixtures of the mono- and di-species but involves the extraction of beryllium, not iron; and the solutions extracted are sulfate solutions, not fluoride solutions.
U.S. Pat. No. 2,920,938 (Matoush) teaches that aqueous AlF.sub.3 solutions can be made from an aluminum-bearing material and a fluorine-containing acid followed by crystallization and U.S. Pat. No. 2,155,199 (Ebner) teaches that pyrohydrolysis of aluminum fluoride crystals yields Al.sub.2 O.sub.3 and HF.
No prior art is known which teaches or suggests the use of mixtures of mono-alkylphenyl and di(alkylphenyl) phosphoric acids for removing dissolved ferric iron from aluminum fluoride solutions. Quite to the contrary, the Marcus et al text, the Grunig et al I patent and U.S. Pat. Nos. 3,835,214 (Hurst et al) and 3,514,266 (Nichols et al), teach that aluminum instead of, or in addition to, iron is extracted when various other types of organo-phosphoric acids are used as extractants in aqueous solutions containing aluminum salts other than the fluoride. There is no teaching leading the skilled worker to use the organic extractant mixtures, disclosed and claimed herein, to selectively extract ferric iron from aluminum fluoride solutions containing dissolved iron impurities.